The performance requirements for gas turbine engines are continually being increased to improve engine efficiency, necessitating higher internal operating temperatures. Thus, the maximum operating temperatures of the materials used for components in these engines, particularly turbine rotor components such as turbine disks, continue to rise. Components formed from powder metal (P/M), precipitation strengthened .gamma.' Ni-base superalloys can provide a good balance of creep, tensile and fatigue crack growth properties to meet these performance requirements. Typically, P/M .gamma.' Ni-base superalloys are produced by consolidation of superalloy powders, using methods such as extrusion consolidation. These consolidated P/M superalloys are used to make various forging preforms. Such preforms are then isothermally forged into finished or partially finished forms, and finally heat treated above the .gamma.' solvus temperature to control the grain size and .gamma.' distribution. Methods for consolidation of P/M superalloys and the creation of preforms are well known.
With respect to .gamma.' Ni-base superalloys, isothermal forging is a term that is used to describe a well-known forging process that is done at slow strain rates (e.g. typically less than 0.01 s.sup.-1) and temperatures slightly below the .gamma.' solvus temperature (e.g. &lt;100.degree. F.), but above the recrystallization temperature of the particular superalloy. These processing parameters are chosen mainly to foster superplastic deformation, which in turn results in low forging loads and low die stresses during forging. Isothermal forging requires expensive tooling, an inert environment, and slow ram speeds for successful operation. Superplastic deformation in the workpiece allows large geometric strains to be achieved during the forging operation without causing cracking within the forging. At the end of an isothermal forging operation, no substantial increase in dislocation density should be observed, as swain is accommodated by grain boundary sliding and diffusional processes. In the event that dislocations are generated, the high temperatures and slow stroke rates allow dynamic recovery to occur. Thus, this forging method is intended to minimize retained metallurgicai strain at the conclusion of the forming operations. Isothermal forging is known to produce a uniform, fine average grain size, typically on the order of ASTM 12-14 (3-5 .mu.m). Reference throughout to ASTM intercept or ALA grain sizes is in accordance with methods E112 and E930 developed by the American Society for Testing and Materials, rounded to the nearest whole number. For applications that demand enhanced creep and time dependent fatigue crack propagation resistance, coarser grain sizes of about ASTM 6-8 (20-40 .mu.m) are required. These coarser grain sizes are currently achieved in isothermally forged superalloys by heat treating above the .gamma.' solvus, but below the incipient melting temperature of the alloy. After isothermal forging and supersolvus heat treatment, cooling and aging operations are also frequently utilized to control the .gamma.' distribution. However, isothermal forging does have some limitations with respect to controlling the grain size of the forged articles.
While isothermal forging tends to produce a ASTM 12-14 (3-5 .mu.m) average grain size, subsequent supersolvus annealing causes the average grain size to increase in a relatively step-wise fashion to about ASTM 6-8 (20-40 .mu.m). Thus, it is generally not possible to control the average grain size over the entire range of sizes between about ASTM 6-14 (3-40 .mu.m) using a single forging method, which control may be very desirable to achieve particular combinations of alloy properties, particularly mechanical properties. Isothermal forging processes are relatively slow forming processes compared to other well-known forging processes, such as hot die or hammer forging processes, due to the slow strain rates employed. Isothermal forging typically requires more complex forging equipment due to the need to accurately control slow strain rate forging. It also requires the use of an inert forging environment, and it is also know to be difficult to maintain thermal stability in many isothermal forges. Therefore, components formed by isothermal forging are generally more costly than those formed by other forging methods.
In addition, unless isothermal forging processes are very carefully controlled, it is possible to impart retained strain into the forged articles, which can in turn result in critical grain growth during subsequent heat treatment operations. Complex contoured forgings contain a range of localized strains and strain rates. If forging temperatures are too low, or local strain rates are too high, diffusional processes that prevent strain energy from being stored in the microstructure cannot keep up with the imposed strain rate. In such cases, dislocations are generated causing strain energy to be retained within the microstructure. As used herein, the term "retained strain" refers to the dislocation density, or metallurgical strain present in the microstructure of a particular alloy. When working a superalloy at temperatures that are less than the alloy recrystallization temperature, the amount of retained strain is directly related to the amount of geometric strain because diffusional recovery processes in the alloy microstructure occur very slowly at these temperatures. However, the amount of retained strain that occurs in a superalloy microstructure that is worked at temperatures that are above the recrystallization temperature is more directly related to the temperature and strain rate at which the deformation is done than the amount of geometric strain. Higher working temperatures and slower strain rates result in lower amounts of retained strain.
When Ni-base superalloys that contain retained strain are subsequently heat treated above the .gamma.' solvus, critical grain growth (CGG) may occur, wherein the retained strain energy in the article is sufficient to cause limited nucleation and substantial growth (in regions containing the retained strain) of very large grains, resulting in a bimodal grain size distribution. Critical grain growth is defined as localized abnormal excessive grain growth to grain diameters exceeding the desired range, which is generally up to about ASTM 2 (180 .mu.m) for articles formed from consolidated powder metal alloys. Critical grain growth can cause the formation of grain sizes between about 300-3000 .mu.m. Factors in addition to dislocation density and retained strain, such as the carbon, boron and nitrogen content, and subsolvus annealing time, also appear to influence the grain size distribution when critical grain growth occurs. Critical grain growth may detrimentally affect mechanical properties such as tensile strength and fatigue resistance.
The affect of retained strain on the final grain size in forged Ni-base superalloys has been described, for example, in U.S. Pat. No. 4,957,567, which is herein incorporated by reference. Applicants have also obtained data from tests described herein that measure grain size as a function of room temperature compressive strain following supersolvus annealing, as shown in FIG. 1. FIG. 1 summarizes the CGG characteristics for the P/M .gamma.' Ni-base superalloy Rene' 88DT. Analogous behavior has been observed in Rene' 95, and is known to occur in cast and wrought superalloys and other alloy systems. This CGG behavior after room temperature deformation may be translated to predict CGG behavior due to elevated temperature deformation; however, strain rate and temperature then replace strain as the primary variables that influence the amount of retained strain. Generally, for P/M .gamma.' superalloys, there is a range of slow strain rates and corresponding forging temperatures in which critical grain growth can be avoided, thus producing a microstructure of uniform grains having an average grain size of ASTM 6-8 (20-40 .mu.m) after supersolvus heat treatment. This range is roughly 0.01 s.sup.-1 or slower, at forging temperatures that are 0.degree.-200.degree. F. below the solvus temperature. It would be desirable to forge well below 0.01 s.sup.-1 in order to avoid the potential for CGG but this is not practical from a productivity standpoint.
Critical grain growth is thought to result from nucleation limited recrystallization followed by grain growth until the strain free grains impinge on one another. The resulting microstructure has the bimodal distribution of grain sizes noted above. As illustrated in FIG. 1, CGG occurs over a relatively narrow range of retained strain. Slightly higher retained strain results in a higher nucleation density and a finer and more homogeneous resultant grain size. Slightly lower retained strain is insufficient to trigger the recrystallization process. Thus, the term critical grain growth was adopted to describe the observation that a critical amount or range of retained swain was required to lead to this undesirable microstructure.
Critical grain growth is not observed in Ni-base superalloys containing a high volume fraction of .gamma.' until heat treatment is performed above the .gamma.' solvus. It is therefore noted that, in this complicated alloy system, factors in addition to retained strain influence grain structure evolution. Particles that pin grain boundaries play an active role in controlling grain size, most notably, the coherent, high volume fraction .gamma.' phase. Carbides, borides and oxides are also reported to influence final grain size, especially if the alloy is heat treated above the .gamma.' solvus.
An alternative procedure to high temperature-low strain rate, isothermal forging is to forge Ni-base superalloy components at higher strain rates and lower temperatures, such that the retained strain everywhere is greater than the critical amount, and above the range that would lead to critical grain growth. This approach is also described, for example, in U.S. Pat. No. 5,413,572, which is incorporated herein by reference. The method described involves forging to achieve high retained strain, followed by supersolvus annealing to recrystallize the microstructure. The grain sizes obtained were described as being in the range of about ASTM 2-9 (15-180 .mu.m) for article formed from P/M forging preforms.
However, it is desirable to develop additional forging methods for these Ni-base superalloys, particularly methods that permit more control over the grain size of the microstructure in the range of ASTM 5-14 (3-60 .mu.m) than present forging methods, and specifically methods that provide control over a broader range of these grain sizes, so as to facilitate the production of forgings having a fine, uniform grain size, while also avoiding CGG.